Experimental melting of hydrous low-K tholeiite: evidence on. the origin of Archaean era tons

Abstract

Experimental melting studies were performed on a natural high-Al basalt and a synthetic average Archaean tholeiite (AAT) composition (0.3 wt.% K20) with variable amounts of H20. Microprobe analyses of quenched melts (glass) from runs at 5-30 kbar and 750°-l100°C showed that typical Archaean tonalitic and trondhjemitic "grey gneiss" compositions were produced from the average Archaean tholei-ite over the entire experimental range, with 15% to less than 1 % H20. The high-Al basalt produced liquids too high in Al203 (18--23%) for realistic grey gneiss compositions. The persistent generation in our experiments of low-K calc-alkaline magmas directly by vapor-undersaturated partial melting of low-K Archaean tholeiite strongly suggests this mechanism for the origin of early continents. Temperatures of 850°-l000°C and pressures around 15 kbar are appropriate melting conditions. Ton­alitic magmas are favored by higher temperatures, lower pressures, and higher H20 contents in the source. Trondhjemitic magmas are favored by lower temperatures, higher pressures, and lower H20 contents. Heavy REE depletion of magmas would be possible for partial melting above 15 kbar because of the stability at higher pressures of residual garnet. Unfractionated REE patterns of magmas could result from melting at lower pressures, where garnet does not coexist with liquid. The low-K trends of melts are maintained by very refractory amphibole (up to 0.7 wt. % KzO) which coexists with liquid for bulk H20 contents of 2 wt. % or more. Shallow subduction-zone melting of amphibolite with magma extraction, and partial melting of amphibolite under deep-crustal metamorphic conditions are models for early crustal evolution which appear to satisfy the experimental constraints.